光电导开关（精选5篇）
光电导开关 篇1
光电导开关 (Photoconductive Switches) , 全称为光控光电导半导体开关 (Optically Controlled Photoconductive Semiconductor Switches-PCSS 或 Bulk Optically Controlled Semiconductor Switch-BOSS) 。光电导开关的非线性特性在高功率超宽带脉冲 (电磁导弹) 产生, 在超快电子学、高功率微波发生器、高功率微波武器、雷达和通信等领域有极为广阔的应用前景和重大意义[1]。人们对光电导开关的工作机理进行了深入而广泛的研究, 已有成熟的理论解释其线性特性的工作机理。但其非线性特性的工作机理复杂, 虽已进行了大量的研究, 也提出了很多的理论, 但迄今为止仍没有一种理论能全面的解释实验中所观察到的各种现象, 光电导开关非线性机理出现的工作条件也不十分清楚。文中实验研究了光电导开关产生非线性的电阈值条件, 并探讨了电阈值条件变化的物理机制。
1 光电导开关 (PCSS) 的基本原理
1.1 光电导开关的结构
光电导开关的结构, 如图1所示, EL2:GaAs半导体基片有一对平行欧姆接触的电极, 电极之间留有宽度约1 mm缝隙。电极与微带传输线和同轴电缆相连形成开关的输入输出端, 即光电导芯片的一端通过一段微带传输线与一直流电源相连形成直流偏置, 另一端通过微带传输线与一匹配负载相连。当激光脉冲照射到直流偏置的光电导开关的电极缝隙上时, 光电导材料吸收入射光子, 使价带或深层能级的电子激发到导带, 开关内部将产生自由运动的光生载流子, 载流子在偏置电场的作用下定向运动, 光电导开关导通。
光电导开关有两种类型:横向光电导开关和纵向光电导开关。横向开关的激励光脉冲入射方向与偏置电场方向相互垂直;纵向开关中激励光脉冲入射方向和偏置电场方向平行。由于横向光电导开关具有较大的光照面积和电导通道, 实验研究采用横向光电导开关[1]。
1.2 光导开关的两种工作模式
当激励光脉冲能量和偏置电压中的任一参量低于其阈值条件时, 光电导开关输出电脉冲与激励光脉冲具有相似的波形, 即呈线性变化关系, 称光电导开关的这种工作模式为线性工作模式。在线性工作模式下, 开关的闭合与关断由光脉冲时变特性决定, 激励光脉冲产生的光生载流子的时变特性决定开关的工作状态[2]。光电导开关输出电脉冲的上升时间主要取决于触发光脉冲的上升时间, 其下降时间主要取决于光导材料介质弛豫时间和光生载流子的寿命以及载流子渡越开关间隙所消耗的时间。图2是在直流偏置电压为200 V, 用平均功率45 W, 脉宽为113 ns的半导体激光器触发光电导开关产生的线性电脉冲波形, 脉冲脉宽4.6 ms, 幅值162 mV。
当激励光脉冲能量和偏置电压参量同时高于某阈值条件时, 光电导开关一旦导通, 即使激励光脉冲消失, 只要偏置电压维持在某一高压, 光电导开关仍处于导通状态。此时开关的工作状态不再由激励光脉冲时变特性决定, 激励光脉冲只起触发开关导通的作用, 这种工作模式即为光电导开关的非线性工作模式。偏置电压为2 000 V, 50 dB信号衰减时, 用平均功率45 W, 脉宽为113 ns的半导体激光器触发开关得到的非线性电脉冲波型, 如图3所示。非线性电脉冲脉宽0.32 μs, 幅值4.1 V。
2 实验研究
实验中的光电导开关用电阻率大于108 Ω·cm的EL2 补偿高阻砷化镓 GaAs (EL2:GaAs) 材料制成, 电极间隙为1.5 mm。它并非有意掺杂, 而是本身具有EL2 缺陷, 其浓度一般为1×1016～2×1016 cm-3[3]。实验装置的基本结构, 如图4所示。激励光源为半导体激光器, 可输出脉冲间隔可调的十个脉冲组成的脉冲串, 输出波长904 nm, 峰值功率45 W, 单脉冲宽度100 ns左右。JAY-2A型直流高压电源作为光电导开关的直流偏置, 输出电压在100～5 000 V可调。光电导开关输出的电信号经50 dB衰减, 由LECROY 6100示波器观察, 其采样速率1 GHz。观测到的线性和非线性波形, 如图5所示。
为了研究光电导开关非线性产生的电阈值特性, 实验中保持触发激光脉冲能量、脉宽、脉冲串长度和脉冲间隔不变, 实验研究直流偏置随非线性输出次数之间的关系, 并实际测量比较产生非线性输出前后光电导开关的暗电阻。
(1) 实验中首次出现非线性电脉冲直流偏置电压为2 180 V, 产生电信号经50 dB衰减器衰减, 如图6所示。关闭电源及一切设备后, 等待若干小时再次实验, 再次出现非线性电脉冲直流偏置电压降低到1 750 V, 产生的电信号经50 dB衰减器衰减, 如图7所示。重复上述过程, 在更低的直流偏置电压1 630 V下, 产生非线性电脉冲经50 dB信号衰减, 如图8所示。3次试验均采用连续10个脉冲的激光脉冲串触发光电导开关, 脉冲间隔为1 μs。实验表明, 同一光导开关非线性电脉冲会随着电阈值的降低而出现;
(2) 基于上述实验, 又对不同的光导开关重复上述实验过程, 同样观测到产生非线性电阈值不断降低的现象。为探究电阈值降低的原因, 对产生非线性前、后光电导开关的暗电阻进行参数测定。图9为试验前光导开关的伏-安特性曲线。图10是相同的实验条件下共出现6次非线性现象试验后的伏-安特性曲线。图11为试验前、后的伏-安特性对比曲线, 实线为试验前特性曲线, 虚线为实验后特性曲线, 两者对比可见试验后暗电阻明显低于试验前。
3 实验结论
暗电阻的降低属于一种损伤, 激光与半导体光电材料相互作用以及非线性的产生都会使材料表面和其内部发生一系列物理、化学变化。
(1) 激光反复触发光导开关时砷化镓 (GaAs) 材料吸收激光并积累热量, 材料吸收光能转化成热量使得材料内部温度不断升高。反复的激光触发会引起材料表面形态发生变化, 原来光滑平坦的材料表面变得粗糙扭曲, 材料表面形态开始发生变化、引发损伤, 从而使开关性能发生改变;
(2) 在制备和加工过程中, 由于存在制备加工水平有限, EL2 补偿高阻砷化镓 GaAs材料中存在大量的微观缺陷, 这些微观缺陷具有比材料本征吸收大得多的吸收率, 导致材料损伤阈值降低。在材料与激光脉冲相互作用过程中, 缺陷吸收占主导地位使材料内形成局部高温, 热量积累达到一定程度就会造成材料缺陷处发生热损伤过程;
(3) 光电导开关工作于非线性模式时, 通过对其发射的红外荧光拍摄图像可以观测到开关中存在电流丝, 实验测得电流丝的传播速度约为 (2±1) ×109 cm/s, 是室温下高纯GaAs中电子最大漂移速度的100倍。电流丝中载流子浓度约为5×1018 cm-3[4], 高电流密度的电流丝必然有高的热效应, 给光电导开关带来内部损伤。
参考文献
[1]Mikulics M, Zheng X, Adam R, et al.High-Speed Photoconductive Switch Based on Low-Temperature GaAs[J].IEEE Photonics Technology Letters, 2003, 15 (4) :528-530.
[2]林维涛, 阮成礼, 杨宏春.光导开关产生超短电脉冲的实验研究[J].电子科技大学学报, 2003, 32 (6) :131-134.
[3]Amit Garg, Avinashi Kapoor, Tripathi K N.Laser-in-duced Damage Studies in GaAs[J].Optics&Laser Technology, 2003, 35 (1) :21-24.
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光电导开关 篇2
Photoconductive Semiconductor Switches(PCSS)have been widely used in pulse power equipments,and they have several significant advantages over other high voltage switches,involving easy fabrication,zero device-to-device jitter for multiple elements,and complete electrical isolation for trigger.(see Fig.1)Improvements have been made in the last ten years in the realms of reducing the dark current on the surface of the semiconductor and enhancing the tolerance of ultra short pulses with the risetime in ns range[1,2,3,4].However,the surface flashover of the semiconductors in the switches is still a restriction to the performance of PCSS,and in order to improve the pulse voltage and energy density transmitted through the PCSS,the mechanism of surface flashover of solid materials in vacuum has to be clarified.In the previous researches,the Secondary Electron Emission Avalanche(SEEA)model[5]is proposed and is wildly accepted in explaining numerous experimental results.Yet,in this model,the effect of the magnetic field generated by the rapid changing electrical field between two electrodes is not taken into account,but with the development of ultra short pulses bearing rise times from several tenths of ns to several ns,the self-initiated magnetic field is large enough to resist the flashover process,therefore in these cases this kind of effect has to be considered.So that a renewed SEEA model is proposed,and the fast variation of electrical field is taken into account,thus the effect of the self-initiated magnetic field is included in this model.According to the result of this modification,the duration of the pulse is inversely proportional to the flashover voltage of the material,and this relationship is considerably in accordance with the experimental consequences illustrated by several articles.A comparison is made between the theoretical predictions and experimental results,and the differences between them are analyzed in detail.
1 Surface Flashover Model
1.1 Summary
High-voltage electrical breakdown at solid dielectric vacuum interfaces is a problem of great importance in a large number of pulsed power situations in which both high voltage and very short rise times are required.And to demonstrate the mechanism of this breakdown process,a sequence of events is applied to show the formation of the flashover phenomenon.First,when the high electrical voltage is applied,the junction of the cathode,solid material,and the vacuum is stimulated andelectrons are projected from this junction;as the electrons drifting to the anode,they collide to the surface of the material again,and more secondary electrons are stimulated by this collision.When the emission of secondary electrons reach the unity energy A1,the secondary electron avalanche is formed.Then,the electron current transmits across the surface between the cathode and anode in a line,and as a consequence of the electron bombardment of the insulator surface,neutral and ionized atoms and molecules are released from the gases absorbed on the surface by electron-stimulated desorption.Some of the desorbed neutrals are ionized when they drift through the layer of electrons,then as the ions accumulate above the electron layer,the field near the cathode is enhanced,which in turn enhances the gas desorption and electron emission,and these regenerative processes finally generate the breakdown[6].
1.2 Surface Charging and Electron Fluxes
As the secondary emission electrons drifting toward the anode,layers of avalanche electrons,neutrals,and positive ions are formed,so that a component of electric field which is perpendicular to the surface of the dielectric is initiated.The layers of different particles are shown in Fig.2,and the formation of the avalanche process is shown in Fig.3.Since the density of surface charge is steady in the prebreakdown period,so that the magnitude of the perpendicular surface electric field remains unchanged,thus the electric field above the surface is inclined at a critical angleθrespect to the dielectric surface.The angle is given by
where is the mean energy of the emission electrons,and tan θ=Er/Ez=η[7].And according to the experimental observation,θranges from approximate 16°to 40°.A1 is the unity energy of the secondary electron.Then the positive surface charge is
We also make the assumption that the magnitude of the negative surface charge is equal to that of the positive surface charge,since the electrons flux diminishes along its path,and the enhancement of electron emission made by the positive charge can compensate for this loss,which isσ+=σ-.By this assumption,the perpendicular electric field component is zero outside the electron avalanche layer,and Ez is uniform through the whole surface,equal to E.The velocity of the electron drift is ve=2A1/me,and the velocity of the emission electron is veo=2 /me. As A1 is a few tens of eV and is a few eV,so ve is as fast as 107 m/s,while ve0 has a value of approximate 106m/s.If the solid has a length in the range of several mm,the electron drift will take less than a ns to cross it,which is insignificant compared to the prebreakdown time,so that the prebreakdown period can be assumed to be equilibrium.In this case,the electron flux parallel to the surface is
And on the average,the electrons plunge back to the surface after travel a distanceδ=A1/e E,so the flux perpendicular to the surface is
1.3 Characteristic Delay Time
As neutrals are released from the absorbed gas in the surface of dielectric,according to(4)the flux of desorbed particles is
where the square of parallel electric field is inversely proportional to the prebreakdown duration.
1.4 Magnetic Field Effects
For a single electron which is projected from the angleαto the normal line of the surface,if a stationary magnetic field B is imposed to the surface of the solid dielectric,and the direction of the magnetic field is perpendicular to the electric field and parallel to the surface,the trajectory of the electron is[8]
If the magnetic field is large enough,the breakdown can be prevented,in this situation u(t0=0)andu&(τ0)=0,which means that the electron projected from the triple junction can not collide the surface,thus no extra secondary electron can be generated;however,considering about the fact that the emission electron has an angular distribution,numerical method should be used to calculate the magnitude of the critical magnetic field.The energy enhancement generated by the angular distribution is
whereh=8b/3η2.The relationship betweenβand h is illustrated in Fig.4[8],some of the consequences in the figure is obtained numerically.We can define the function
Since g is a constant,and G has its minimum value with the variation of.If g<Gmin,the magnetic field is strong enough to prevent the intersection of any trajectories.So we can obtain the critical magnetic field through this method.According to Fig.3,the critical value is obtained when h=1,and for magnetic field weaker than this critical one,0<h<1,the surface positive charge has to be larger in order to be strong enough to attract theelectron against the resistance of the magnetic field.And the consequence is that the flashover can be prevent within a certain period of delay time before the final breakdown.And is the length of the pulse is shorter then this period,the flashover can be inhibited.From Fig.4,we can fit the 0<h<1 part of the curve into
This curve fitting has an R-square of 0.999 4.So that,according to the definition of h,the restraint that g is less than 0.3,andη=tanθwhereθis the angle between perpendicular electric field and the parallel one ranging from 16°to 40°,it can be obtained that the relation between the imposed magnetic field and parallel electric field is
where h depends on g andη,it can be calculated numerically from(9).
1.5 The Effect of the Self-initiated Magnetic Field
In this part,the writer develops a simple revised model of SEEA.First,the mechanism of self-initiated magnetic field above a standard cylinder in vacuum is discussed.In respect to the magnitude of the magnetic field,only the magnetic field which is directly generated by the pulsed parallel electric field and the second level magnetic field generated by this varying magnetic field is taken into account,and higher levels of field is omitted due to their insignificant magnitude.So this mechanism is that the first level of electric field initiates the first level of magnetic field,correspondingly the first level of magnetic field generates the second level of electric field,and eventually the second level of electric field produces the second level of magnetic field.
This model consider a parallel electric field generated by a Marx generator as the first level of electric field,and the waveform of the pulse is
where t1 is the risetime of the pulse voltage,and it is in the range of ns,E0 is the peak value of the pulse.So the first level of magnetic field is
where r is the radius of the cylinder,and it has a value of several to tens of cm.The magnetic field is applied only to the surface electric layer;if the parallel electric field is approximate several MV/m,the magnitude of the first level magnetic field is in the range of 0.1 T.And the second level magnetic field is
Then
Under the above parameters,the second level magnetic field is in the range of 0.01 T.The whole effective magnetic field is
This effect is illustrated in Fig.5.When the risetime of the pulse is set as 1 ns,the peak voltage is set as 10MV/m,the magnetic field in the vicinity of the semiconductor surface is shown in Fig.6.
1.6 The Relationship between the Pulse Rise Time and Flashover Voltage
According to(2)and(11)
With(21)we obtain the relationship between surface charge which resists the effect of magnetic field and the self-initiated magnetic field:
This is a very important relationship between the prebreakdown duration(that is the time between the pulse reach its 90%of magnitude and the time of breakdown)and the magnetic field,sinceτserves as a criteria for the magnitude of B,that is when the magnetic field overrides this such critical point determined by this equation,can the prebreakdown duration reach that extend.So,according to the expression of the self-initiated magnetic field,(2)and(9),the breakdown electric field of is
This is the expression of the flashover electric field taking into account of the self-initiated magnetic field.The risetime of the pulse is in general inversely proportional to the breakdown electric field.
2 Comparison with experimental results
Fig.7 illustrates the breakdown electric field versus the risetime of the applied pulse voltage.Since all conventional dielectric materials have g≤0.3,we set g=0.1,η=0.48,ηh2=0.022 4.The radius of the cylinder r is setseparately as 5 cm,10 cm,and 15 cm,the emission electron velocity ve0 is set as 107 m/s.The experimental results are shown in Fig.8(b),a chain of consequences abstracted from several experimental articles are combined together to show the relationship of pulse risetime versus the breakdown electric field.
If the magnitude of E0 is 10 MV/m,in that the voltage between two electrodes can reaches 0.2 MV if the semiconductor block has a length of 10 cm.Fig.8(a)illustrates schematically the flashover voltage varying with the risetime,where the radius ris set to be 15 cm.The tendency shown in the theory calculation in general fits that of the experimental results despite some of the parameters is different between two figures,which is due to the selection ofθand g.However the disadvantage of this model is that it assumes that the electric field is uniform throughout the whole dielectric surface,and the regenerative events of the ionized particles are unable to break this situation,and the calculation of the self-initiated magnetic field doesn't take into account the affect of the shape distribution of solid dielectric and assumes that the working material is a standard cylinder,while the fact is different shape of dielectric process different resistance to the flashover,and the distribution of the magnetic field is also affected by the surface shape So the special design of shape is a critical factor in improving the performance of dielectrics.It can also be observed that the resistance of dielectric experiences a nadir when the risetime of pulses reach the range of ms then the flashover voltage rises until the pulse transforms into a dc voltage.
The tendency shown in the theory calculation in general fits that of the experimental results,despite some of the parameters is different between two figures,which is due to the selection ofθand g,and the nature of material However,the disadvantage of this model is that it assumes that the electric field is uniform throughout the whole dielectric surface,and the regenerative events of the ionized particles are unable to break this situation,and thecalculation of the self-initiated magnetic field doesn't take into account the affect of the shape distribution of solid dielectric,and assumes that the working material is a standard cylinder,while the fact is that different shapes of dielectric possess different resistances to flashover,and the distribution of the magnetic field is also affected by the surface shape.So the special design of shape is a critical factor in improving the performance of dielectrics.It can also be observed that the resistance of dielectric experiences a nadir when the risetime of pulses reach the range of ms,then the flashover voltage rises until the pulse transforms into a dc voltage.
3 Conclusions
A simple revised model based on the SEEA model is developed,and it predicts the tendency of breakdown electrics field versus the risetime of the applied pulse.Therefore,to improve the performance of PCSS under the ultra short pulsed voltage.From Fig.7,the larger the radius of the semiconductor,the larger flashover voltage the semiconductor can hold.The geometry of the semiconductor can also be revised to withstand higher voltage[11,12]After clarifying the relationship between the risetime and the applied electric field,we can better know the fundamental characteristics of high voltage equipments,thus more developments can be made upon it.
参考文献
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光电导开关 篇3
关键词：光电式接近开关,行程限位,检测距离
1 概述
多年来起重机械因行程限位失效, 造成运行机构撞击、金属结构变形, 甚至造成严重事故的现象屡见不鲜。这是因为机械式限位开关的金属传动部件长年暴露在外, 受周边环境侵蚀, 易发生故障。如限位开关的开关臂轴与轴孔因缺油而锈蚀, 无法转动;限位开关的开关臂在扳动后不能自动复位, 运行机构无法正常第二次运行;因安全尺变形, 无法使限位开关动作;因限位开关的进线孔不密封, 造成潮气侵入, 使其内部的电气触点氧化, 造成触点接触不良;限位开关内部的回位弹簧因锈蚀变形, 失去原有作用, 造成触点接触不良及不能自动复位。
因接触式限位开关有如上缺陷, 如果采用非接触式限位开关, 就能避免上述缺陷。经对比分析, 选择了用接近开关来代替接触式限位开关。
接近开关是一种毋需与运动部件进行机械接触而可以操作的位置开关, 又称无触点接近开关, 是理想的电子开关量传感器。当检测体接近开关的感应区域, 不需要机械接触及施加任何压力, 开关就能无接触, 无压力、无火花、迅速发出电气指令, 准确反应出运动机构的位置和行程。
2 接近开关特点
与接触式限位开关相比, 接近开关具有以下特点:
2.1 非接触检测, 不影响被测物的运行工况;
2.2 不产生机械磨损和疲劳损伤, 工作寿命长;
2.3 响应快, 一般响应时间可达几毫秒或几十毫秒;
2.4 采用全密封结构, 防潮、防尘性能较好, 工作稳定性强;
2.5 无触点、无火花、无噪音, 适用于要求防爆的场合;
2.6 输出信号大, 易于与计算机或可编程控制器 (PLC) 等接口;
2.7 体积小, 安装、调整方便。
3 接近开关原理及分类
接近开关根据原理不同, 可以分为以下几种:
3.1 涡流式接近开关
这种开关有时也叫电感式接近开关。它是利用导电物体在接近这个能产生电磁场接近开关时, 使物体内部产生涡流。这个涡流反作用到接近开关, 使开关内部电路参数发生变化, 由此识别出有无导电物体移近, 进而控制开关的通或断。这种接近开关所能检测的物体必须是导电体。
3.2 电容式接近开关
这种开关的测量通常是构成电容器的一个极板, 而另一个极板是开关的外壳。这个外壳在测量过程中通常是接地或与设备的机壳相连接。当有物体移向接近开关时, 不论它是否为导体, 由于它的接近, 总要使电容的介电常数发生变化, 从而使电容量发生变化, 使得和测量头相连的电路状态也随之发生变化, 由此便可控制开关的接通或断开。这种接近开关检测的对象, 不限于导体, 可以绝缘的液体或粉状物等。
3.3 霍尔接近开关
霍尔元件是一种磁敏元件。利用霍尔元件做成的开关, 叫做霍尔开关。当磁性物件移近霍尔开关时, 开关检测面上的霍尔元件因产生霍尔效应而使开关内部电路状态发生变化, 由此识别附近有磁性物体存在, 进而控制开关的通或断。这种接近开关的检测对象必须是磁性物体。
3.4 光电式接近开关
利用光电效应做成的开关叫光电开关。将发光器件与光电器件按一定方向装在同一个检测头内。当有反光面 (被检测物体) 接近时, 光电器件接收到反射光后便在信号输出, 由此便可"感知"有物体接近。
3.5 其它型式接近开关
当观察者或系统对波源的距离发生改变时, 接近到的波的频率会发生偏移, 这种现象称为多普勒效应。声纳和雷达就是利用这个效应的原理制成的。利用多普勒效应可制成超声波接近开关、微波接近开关等。当有物体移近时, 接近开关接收到的反射信号会产生多普勒频移, 由此可以识别出有无物体接近。
4 光电式接近开关
相比其它类型的接近开关, 光电式接近开关具有检测距离长的特点, 现起重机械上非接触式限位开关多使用反射器型光电开关。
4.1 主要参数
检测距离:是指检测体按一定方式移动, 当开关动作时测得的基准位置 (光电开关的感应表面) 到检测面的空间距离。
4.2 工作原理
由振荡回路产生的调制脉冲经反射电路后, 由发光管辐射出光脉冲。当被测物体进入受光器作用范围时, 被反射回来的光脉冲进入光敏三级管。并在接收电路中将光脉冲解调为电脉冲信号, 再经放大器放大和同步选通整形, 然后用数字积分或RC积分方式排除干扰, 最后经延时 (或不延时) 触发驱动器输出光电开关控制信号。光电式接近开关作为桥式起重机非接触式行程限位, 将发光管和接受器 (一体式) 固定在端梁端部 (或小车架端部) , 反射板固定在大车轨道端部 (或小车轨道端部) 对应位置, 当大车 (或小车) 向轨道端部移动, 移动到检测距离时, 接受器收到经反射板反射的光线, 经传感器, 控制电路分析处理, 输出信号给相应控制回路, 停止该方向运行。
5 注意事项
5.1 从行程限位动作失电到相应机构停止, 需要一段时间, 也需要一段减速距离, 因此, 在选择光电式接近开关时, 检测距离必须大于减速距离。在平时检验时, 已发现几起光电式接近开关检测距离小于减速距离, 导致大车制停距离不足, 撞上大车止档事件。
5.2 强光会对光电式接近开关产生干扰, 户外或太阳光等有强光直射场所应做好遮光措施。
6 结束语
光电式接近开关作为起重机械的行程限位, 避免了接触式限位开关的劣势, 给安全生产带来了可靠保障, 并且光电式接近开关安装方便, 使起重机械在使用的可靠性、安全性及维修的方便性等方面均得到很大提高。同时给起重机械的电气系统PLC控制的配套运用打下了良好基础, 为起重机械电气自动化发展做好准备,
参考文献
[1]王福绵.《起重机械技术检验》.北京:学苑出版社, 2000
光电导开关 篇4
关键词：行走数据,计数,识别,校正
一、概述
秦皇岛港煤四期项目 (即秦皇岛港务股份有限公司第七分公司) 是国家“八五”、“九五”期间的重点工程。港口大型装卸机械主要有翻车机、堆料机、取料机、装船机、堆取料机等设备。堆料机通过驱动装置, 沿轨道行走。其行走方式与大多数港口装卸机械设备基本一致。堆料机的行走数据一般是通过PLC程序计算一个与轨道相接触的测距轮的运转情况而得出的。堆料机在经过多次的往复行走作业过程, 大机的行走数据有时会出现误差, 可能造成与相邻港口装卸机械之间碰撞的危险。目前, 光电设备以其强抗干扰能力, 在生活和工业生产过程中, 起着至关重要的作用。而将光电子设备与机械故障检测及数据校对相结合所起到的效果已经比较明显。因此, 将引入光电开关作为数据校正设备, 对港口机械行走过程中产生的数据实现灵敏度较高的校正。
光电开关是把发射端和接收端之间光的强弱变化转化为电流的变化以达到探测的目的。由于光电开关输出回路和输入回路是电隔离的 (即电缘绝) , 精度高、响应速度快、检测距离远, 使其在机械测量、液位控制以及安全防护等诸多领域广泛应用。
二、具体实施方法
在堆料场的特定位置指定为机械行走数据校正点, 装卸机械设备通过某校正点处, 堆料机的PLC程序可以实现对此校正点的识别、判定, 然后在对应的校正点将行走数据精确地写入事先编译完成的PLC程序。而当线路出现故障时, 在程序中对线路故障做出判断, 暂时中断校正程序。
1. 首先确定数据校正装置。
采用德国西克 (SICK) 公司生产的一种镜反射式光电开关作为数据校正装置。这种装置抗干扰能力突出, 灵敏度高, 有效扫描距离最大可达55米, 可以在环境较为恶劣的现场进行作业。要求有相应的反光板进行配套使用, 这样才可以将光电开关发射的光信号返回堆料机PLC接收装置, 从而进行下一步的程序处理。
2. 要确定行走数据校正点的位置, 需要对光电开关配套使用的反光板的位置进行设定。
港口行业中, 将堆存煤炭的场地会划分成长度、宽度相同的矩形区域作为堆料区。这些矩形区域之间通常会有20~30米宽的分隔区域。在本案中, 我们将这些区域的中点将作为数据校正点, 如图1所示。而反光板的数量也是根据不同的校正点来进行确定。
3. 在堆料机的PLC程序编译过程中, 以限时、计数的方法识别校正点。
即:当载有光电管的堆料机行走至各反光板时, 光电开关接收到反光板反射的射线, 输入点记为“0”, 并作为扫描周期的起始点。当载有光电管的堆料机行走至各反光板之间10cm的间隙时, 光电管无法接收到反射射线, 为“1”。在扫描计时周期结束时, 将计数结果作为中间变量写入到PLC程序。因此, 当堆料机以1m/s的速度行走时, 接收扫描结果的输入点将会产生多个时长为0.1秒的脉冲信号。在PLC程序中, 以计数器来计算脉冲信号的数量。
在一个校正点处连续设置反光板的最大数量为4个, 反光板宽度为5cm, 之间间隔为10cm。堆料机行走扫描经过相邻的4块反光板所用的最长时间, 即扫描周期为: (4×5+3×10) /100/1=0.5s。根据现场测量, 记录每个校正点位置及相应的反光板数量, 当PLC程序的中间变量恰好等于某个数值时, 相应的已测数据将写入到机械行走数据。
4. 当光电开关的线路出现故障时, 程序工作流程图如图3所示。
在程序中设置提示, 提醒维修人员及时处理线路故障。
三、实施效果。
光电导开关 篇5
在航母的整个作战链条中,舰载机的降落是最重要的环节之一,也是事故率最高的环节。舰载机在距舰船尾端1 400 m进入着舰导引窗口,维持一定攻角且追踪对准跑道中心线的状态下,沿与水平面成-4°夹角的理想下滑线降落至跑道理想降落点。舰载机必须在甲板指定区域内挂上阻拦索减速,才能在母舰甲板的短距跑道上停降[1,2,3,4,5]。传统菲涅耳灯箱着舰导引方式存在的主要不足是在处于较高等级海情时,舰体的纵摇和沉浮很容易引起跑道1.25 m以上的垂直高度变化,对应着舰点的前后移动超过18 m,这会导致挂阻拦索失败造成复飞甚至酿成事故[6,7]。
在这样背景下,我们考虑用光学导引的方式完成着舰。为了适应更为严峻的气候和物理条件,红外成像技术有更好的成像效果。我们研究了着舰红外合作目标组合,在航母甲板跑道平面上预先设置已知的红外合作目标,进入着舰导引窗口后舰载机的红外系统锁定五个合作目标,得到五个合作目标中心像素坐标,并结合惯导信息及已知的五个合作目标的位置关系由光电导引算法实时解算出飞机相对跑道的运行轨迹、飞机的姿态、跑道的运动规律、着舰点位置等,实现舰载机的光电导引自动着舰。
2 地面车载半实物仿真实验方案
我们用五个红外合作目标来模拟航母跑道,其中四个代表跑道的四个端点,一个位于中心,合作目标所成平面与地面成4°夹角,此时红外成像系统与合作目标平面的角度等同于着舰时飞机与跑道的下滑角。将合作目标置于导轨滑块上,控制导轨运动从而带动合作目标上下运动,模拟甲板跑道因海浪作用在垂直方向上的运动。将红外光电系统置于试验车上探测、识别、锁定合作目标,模拟舰载机从进入1 400 m的下滑窗口,到340 m结束的导引过程。车载半实物仿真实验示意图如图1所示。方案采取对航母甲板跑道和着舰导引距离按5:1的比例缩放。红外系统得到五个合作目标图像信息后,光电导引算法根据系统模型,按以下步骤估计飞机相对跑道的位姿及舰船的运动规律、着舰点等参数:
1)利用惯导系统所提供的俯仰角、横滚角信息对图像进行补偿,可以求出航向角φ。
2)将1)所得航向角φ合作目标中心在摄像机坐标系的二维坐标可求得γs、θs、xcγ、ycγ、(zcγ-hs)。其中γs为航母横摇角,θs为航母纵摇角,(xcγ,ycγ,zcγ)T为摄像机坐标系在跑道坐标系的坐标,hs为航母沉浮量。
3)利用机载惯导信息对飞机高度变化进行补偿,对所求得的γs、θs以及补偿后的相对高度变化进行参数估计,估计出舰船的运动规律。
4)由实时计算所得的飞机相对于舰船的位置和步骤3)所得舰船运动规律,预测飞机着舰点位置。
3 视觉系统的确定及合作目标的设计
3.1 视觉系统的确定
光的大气透过率和大气窗口如图2所示,可以看出,红外辐射透过率高达90%的电磁波段在8～14µm之间,这表明此波段的视觉系统探测距离最远。相关研究也大多采用8～14µm红外视觉系统,因此本文选择了DL S870型8～14µm的红外热像仪作为机载视觉系统。图3为安装在试验车上的红外系统。
3.2 合作目标的研究
1)合作目标功率与热像仪的匹配
舰载机着舰时,与甲板的高度一般为300 m左右,海面上影响红外辐射的主要气体为水蒸汽和二氧化碳。在对红外分子光谱的研究中,已经测得多种分子吸收(发散)光谱谱线和谱带的强度及波数,根据气体分子吸收谱线的波数和强度,可以从理论上计算大气传输中气体分子对红外辐射的吸收[8]。在8～14µm的中远红外波段时,根据文献[8]提供的大气的吸收和散射衰减的模型及海平面上水平路程水蒸汽和二氧化碳在8～14µm波段的光谱透过率,可以近似地计算出功率为172 W的合作目标在L=280 m处接收到的功率足以满足热像仪的器件探测要求。
2)目标-背景的辐射对比度
除了需要考虑大气状况、目标的功率等对红外图像采集的影响,由于背景也存在不同程度的红外辐射。因此,在工程应用时,我们必须还要研究目标和背景之间的相互影响,才能得到与实际符合或相近的结果。
当外界环境条件较差时,越接近于朗伯反射体的目标其光谱辐射出射度越好。为了提高合作目标-背景的辐射对比度,通过加热合作目标,提高目标-背景的辐射对比度,使目标更容易被探测到。辐射源在特定波长上的光谱辐射效率为
式中:MbλT为黑体光谱辐射出射度;c1=2πhc2=3.7418×10-16W.m2,为第一辐射常数;c2=hc/k=1.4388×10-2mK为第二辐射常数;其中,k=.13807×10-23J/K为玻尔兹曼常数;c=.29979×108m/s为光在真空中的传播速度。
当辐射源的光谱辐射率接近1时,辐射源接近于朗伯漫反射,对(1)式中T求极大值得:λeTe=3 369.73µm K。这表明,对于给定波长λe有一对应光谱辐射效率最大的温度Te。则在8～14µm波段范围内,则对应光谱最大辐射效率的温度范围为摄氏温度为152℃～237℃。这与我们选择的180℃相符。
3.3 合作目标大小与红外探测距离的分析
试验车的初始位置距模拟跑道尾端280 m,从此点向模拟跑道(35 m×6 m)行进。红外摄像机为384×288像元。在8°视场角下,当摄像机视场角一定时,合作目标成像像点大小随摄像机与合作目标间距离的减小而增大,故仅需计算最远距离时合作目标大小。若使合作目标在最远实验距离下成像不小于2 pixels×2 pixels大小的图像,则可以计算:
我们选取合作目标的大小为0.3 m×0.3 m。
热像仪能探测到目标所必需的两个条件:一是目标对系统的张角不小于系统的最小可分辨角;二是目标与背景经过大气的衰减作用后的温差不小于系统能探测到的最小温差[9]。在这种情况下,目标为扩展源目标,不能仅仅考虑目标的能量,还要考虑目标的大小、形状。目前对扩展源目标的视距估算,主要以MRTD为评价标准,综合考虑目标、大气的实际状况以及观察等级的要求,观察等级的划分是依据Johnson准则。
MRTD的基本计算公式为
式中:MTF(f)为系统的调制传递函数;te为人眼的时间常数,一般取0.2 s;fp为帧频;fn为等效噪声带宽;τd为驻留时间,SNRDT为阈值显示信噪比;METD为噪声等效温差。
我们根据合作目标的几何尺寸和形状及分辨等级对METD进行修正,修正后的METD′为
式中:n0为约翰逊半周期数;γ为目标的高宽比。
原始温差∆T0经大气衰减后到达系统时的温差∆T为
令∆T=MRTD′,求出此时对应的极限空间频率fT,然后按下式计算作用距离:
根据所用红外摄像机参数,按上述公式估算,对于0.3 m×0.3 m的目标,热像仪的探测距离不小于500 m,大于实验所用280 m,因此所选目标大小能够满足实验要求。
4 实验及结论
在相同距离下,对不同温度的合作目标进行探测。实验发现,随温度升高合作目标更容易从背景中区别出来,在180℃和200℃时,红外合作目标成像效果都比较好,但两者差异不大。考虑到合作目标功率等因素,我们选择最佳辐射温度为180℃。实验结果如图4所示。
将红外合作目标组合的图像数据用于光电导引算法,图5显示飞机在导引过程中实时估计所得的相对航向角误差、相对位置误差。由图可看出,随飞机相对舰船的距离越小估计误差越小,至导引结束时横向(沿Xr轴方向)、纵向(沿Yr轴方向)和垂向(沿Zr轴方向)估计误差分别小于1 m、3 m、1 m,相对航向角误差小于10′。图6显示重复20次仿真实验的导引结束时(基于估计的舰船运动规律)对着舰点的位置预测误差,横向(沿Xr轴方向)、纵向(沿Yr轴方向)和垂向(沿Zr轴方向)估计误差分别小于0.5 m、1 m、1.2 m,其中横向、纵向位置预测结果由于相对航向角的补偿作用,误差减小,垂向位置预测则由于引入舰船运动规律估计误差而略有增大。
摘要：为了实现舰载机全天候自主着舰,本文研究了舰载机光电导引着舰用红外合作目标组合。首先提出了切实可行的工作过程及地面车载验证方案,然后研制了合作目标。根据大气窗口、红外辐射在大气中的传输特性和待选波段对应的最佳辐射温度对合作目标温度控制的影响,确定了8～14μm波段为合作目标对应的辐射波段。根据摄像机的探测距离的估算以及实际要求确定了合作目标的大小。将所设计的合作目标组合用于光电导引算法,计算结果表明,对着舰点的位置预测误差分别小于0.5m、1m、1.2m,具有很高的精度。
关键词：着舰导引,合作目标,红外,舰载机,光电导引
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